This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-228972, filed Jul. 30, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a photosensor system having a photosensor array constituted by two-dimensionally arraying a plurality of photosensors, and an image reading method using the photosensor system.
2. Description of the Related Art
In recent years, individual identification techniques have actively been studied to strengthen the security function in access to a confidential document stored in a computer, e-commerce on a network, and entrance/exit to/from an important facility. The individual identification method uses, e.g., biometric information. In particular, fingerprints are different between individuals, do not change as long as a person lives, and thus are utilized as an important feature which realizes individual identification. From this, fingerprint collation apparatuses have enthusiastically been developed.
This fingerprint collation apparatus requires a two-dimensional image reading apparatus which uses a finger as an object to be sensed and reads a fine three-dimensional structure on the object surface. An example of the two-dimensional image reading apparatus is a photosensor system which comprises a photosensor array constituted by arraying photoelectric conversion elements (photosensors) in a two-dimensional matrix and reads a fingerprint image by the photosensor array. A known photosensor system of this type uses a transmission photosensor as a photoelectric conversion element and has an illumination light source attached to the back surface. A schematic arrangement of the photosensor system is shown in FIGS. 13, 14A, and 14B.
This photosensor system schematically comprises a photosensor array 101 and an illumination light source (back light) BL. In the photosensor array 101, a plurality of photosensors 10 are arrayed in a two-dimensional matrix on one surface of a glass board GB, and a light-receiving surface 102 is formed by covering the photosensor array with a transparent insulating film 20. The light source BL is arranged on the back side of the photosensor array 101 opposite to the side of the light-receiving surface 102. As the photosensor 10, e.g., a CCD is used. The photosensors 10 arrayed in a matrix are scanned and driven by horizontal and vertical scanning circuits (not shown). The number of electron-hole pairs (charge amount) generated in correspondence with the quantity of light incident on the light-receiving portions of the photosensors is detected to sense the luminance of received light.
To read a fingerprint image by the photosensor system, a finger is set on the light-receiving surface 102 and irradiated with light from the light source BL attached to the back surface of the photosensor array_-101. Light emitted by the light source BL irradiates the finger surface through transparent insulating film portions except the formation regions of the photosensors 10. The light is scattered and reflected in accordance with a three-dimensional structure corresponding to the fingerprint of the finger in contact with the light-receiving surface 102. Then, reflected light with a bright/dark pattern corresponding to the fingerprint enters the photosensor array 101.
When the finger touches the light-receiving surface 102, the ridge portions (projecting portions) of the fingerprint tightly contact the light-receiving surface 102, and strongly scatter and reflect irradiation light from the light source BL. A large quantity of light enters the photosensor 10, and the ridge portions are detected bright (white). The valley portions (recessed portions) of the fingerprint do not tightly contact the light-receiving surface 102, and irradiation light from the light source BL is weakly scattered at the interface between the transparent insulating film 20 and air. A small quantity of light enters the photosensor 10, and the valley portions are detected dark (black). The bright/dark pattern corresponding to three-dimensional structure of the fingerprint is two-dimensionally read to read the fingerprint image.
The state of the finger surface, i.e., the state of the skin surface of a person changes due to the individual difference in the secretion of sebum or the moisture retention of the skin. The skin surface state also varies depending on the ambient humidity, air temperature, or the like. The skin surface is finely corrugated, and even the surface of the ridge portion of the fingerprint is subtly corrugated.
FIG. 14A is an enlarged view showing the main part when the skin surface of the finger is moist with a proper amount of sebum or water at the fingerprint reading portion of the photosensor system. FIG. 14B is an enlarged view showing the main part when the skin surface of the finger is dry due to a shortage of sebum or water.
If the finger has a properly moistened skin, as shown in FIG. 14A, the ridge portions can tightly contact the light-receiving surface 102 via sebum, water, or the like. The entire ridge portions strongly scatter and reflect irradiation light, and the ridge and valley portions can be clearly read to accurately read the fingerprint image.
To the contrary, if the finger skin is dry, as shown in FIG. 14B, the skin surfaces at the ridge portions are finely corrugated, which make it difficult to tightly contact the light-receiving surface 102. The projecting portions of the ridge portions tightly contact the photosensor 10, strongly scatter and reflect irradiation light, and are detected bright (white). The recessed portions of the ridge portions do not tightly contact the light-receiving surface and are detected dark (black). In other words, even the ridge portion is detected bright (white) and dark (black) in different parts. As a result, the ridge portion becomes unclear, failing to accurately read the fingerprint image.
The present invention has an advantage that a photosensor system which has a photosensor array constituted by two-dimensionally arraying a plurality of photosensors, uses the fingerprint of a finger as a finger to be sensed, and reads a fingerprint image can clearly read the fingerprint image of even a dry finger, and can read a fingerprint image with a uniform contrast without any contrast nonuniformity generated in the fingerprint image.
The present invention also has an advantage that the power consumption of an illumination light source used can be reduced in reading a fingerprint image.
To achieve the above advantages, a photosensor system according to the present invention comprises a photosensor array which is constituted by two-dimensionally arraying a plurality of photosensors and has a light-receiving surface, a first light source (front light) which is arranged to face the light-receiving surface, and illuminates a rear surface of a finger set on the light-receiving surface, and image reading section which reads a fingerprint image by receiving light that is emitted by the first light source and passes through the finger.
The image reading section in the present invention comprises sensitivity adjustment reading section which reads the fingerprint image at a plurality of image reading sensitivities, optimal image reading sensitivity deriving section which derives an optimal image reading sensitivity suitable for reading operation of the fingerprint image on the basis of fingerprint images read by the sensitivity adjustment reading section at the image reading sensitivities, and image reading sensitivity setting section which sets the optimal image reading sensitivity as an image reading sensitivity. The optimal image reading sensitivity deriving section extracts maximum and minimum values for each image reading sensitivity out of pixel data based on an image pattern of the read fingerprint image, calculates a data range, and derives the optimal image reading sensitivity on the basis of a_-change in the data range for each image reading sensitivity.
In the photosensor system of the present invention, the front light illuminates a finger from its back surface, and light having passed through the finger is received. This can reduce the influence of the contact state between the finger surface and the light-receiving surface of the photosensor array. Hence, the fingerprint image of even a dry finger can be clearly read.
The front light can be a white light source which provides an illuminance of at least 1,000 lux on the light-receiving surface. Since the wavelength of light having passed through the finger mainly falls within the red region, the front light can be a red light source which provides an illuminance of at least 100 lux on the light-receiving surface. In this case, the power consumption of the light source can be greatly reduced.
In order to obtain the above effects, the second photosensor system in the present invention comprises a second light source (back light) which illuminates via the photosensor array the front surface of the finger set on the light-receiving surface, in addition to the first photosensor system. The image reading section reads a fingerprint image by receiving light which is emitted by the first light source and passes through the finger, and light which is emitted by the second light source and reflected by the front surface of the finger.
The use of the back light can compensate for any nonuniformity of the contrast of the fingerprint image which may occur in the use of only the front light, thus obtaining a uniform-contrast fingerprint image.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.